The severity of a stenosis or lesion in a blood vessel may be assessed by obtaining proximal and distal pressure measurements relative to the given stenosis and using those measurements for calculating a value of the Fractional Flow Reserve (FFR). FFR is defined as the ratio of a first pressure measurement (Pd) taken on the distal side of the stenosis and to a second pressure measurement taken on the proximal side of the lesion usually within the aorta (Pa). Conventionally, a sensor placed on the distal portion of a flexible interventional device, such as a guidewire, is utilized to obtain the first pressure measurement Pd, while an external pressure transducer is fluidly connected via tubing to a guide catheter for obtaining the second or aortic (AO) pressure measurement Pa. AO pressure is measured in various coronary catheterisation procedures and typically is measured via an external pressure transducer connected to the proximal end of a guide catheter. Once the guide catheter is positioned in situ, the lumen of the guide catheter fills with blood and the pressure of blood filling the lumen is equal to the pressure of the blood at the distal tip of the guide catheter. Tubing that fluidly connects the proximal end of the guide catheter to the external pressure transducer also fills with blood such that the external pressure transducer records or measures the pressure of the blood. Calculation of the FFR value provides a lesion specific index of the functional severity of the stenosis in order to determine whether the blockage limits blood flow within the vessel to an extent that treatment is needed. An optimal or normal value of FFR in a healthy vessel is 1.00, while values less than about 0.80 are generally deemed significant and in need of an interventional treatment. Common interventional treatment options include balloon angioplasty and/or stent implantation.
Blood flow through the coronary arteries is affected by fluctuations in the pressure arising proximally of the lesion, e.g., in the aorta, as well as fluctuations in pressure arising distally of the lesion, e.g., in the microcirculation. Accordingly, it is not possible to accurately assess the severity of a coronary lesion by simply measuring the pressure differential across the lesion because the pressure measurement taken on the distal side of the lesion is not purely a residual of the pressure transmitted from the aortic end of the vessel. As a result, for an effective calculation of FFR within the coronary arteries, it is necessary to reduce the vascular resistance within the vessel. Currently, pharmacological hyperemic agents, in particular Adenosine, are administered to reduce and stabilize the resistance within the coronary arteries. These potent vasodilator agents reduce the dramatic fluctuation in resistance to obtain a relatively stable and minimal resistance value.
However, the administration of pharmacological hyperemic agents is not always possible or advisable. In some countries, pharmacological hyperemic agents such as adenosine are expensive and time consuming to obtain when delivered intravenously (IV). In that regard, IV-delivered adenosine is generally mixed on a case-by-case basis in the hospital pharmacy. It can take a significant amount of time and effort to get the adenosine prepared and delivered to the operating area. These logistic hurdles can impact a physician's decision to use FFR. In addition, some patients cannot use hyperemic agents due to conditions such as asthma, severe COPD, hypotension, bradycardia, low cardiac ejection fraction, recent myocardial infarction, and/or other factors that prevent the administration of pharmacological hyperemic agents. Further, even if not prohibited, many patients find the administration of pharmacological hyperemic agents to be uncomfortable because vasodilation recreates the symptoms of angina, which is only compounded by the fact that the pharmacological hyperemic agent may need to be applied multiple times during the course of a procedure to obtain FFR measurements.
There is a need in the art for alternative devices and methods for obtaining FFR measurements without the need for a pharmacological hyperemic agent.